1. Field of the Invention
The present invention relates to a semiconductor device, such as a thin film transistor (TFT), using a crystalline semiconductor film formed over a substrate, and to a method of manufacturing the same. The semiconductor device of the present invention includes not only elements such as a thin film transistor (TFT) or a MOS transistor, but also includes liquid crystal display devices, EL display devices, EC display devices, and image sensors having a semiconductor circuit (such as a microprocessor, a signal processing circuit, or a high frequency circuit) structured by these insulating gate type transistors. In addition, the semiconductor device of the present invention includes electronic equipment which is loaded with these display devices, such as a video camera, a digital camera, a projector, a goggle display, a car navigation system, a personal computer, or a portable information terminal.
2. Description of the Related Art
At present, thin film transistors (TFT) are used in various kinds of integrated circuits as semiconductor elements which use a semiconductor film, and in particular are used as switching elements in a pixel region of an active matrix type liquid crystal display device. In addition, in accordance with the high mobility of TFTs, they are also used as driver circuit elements driving the pixel region. It is necessary to use a crystalline semiconductor film, which has a higher mobility than an amorphous semiconductor film, as the semiconductor film used in the driver circuit. This crystalline semiconductor film is called a polycrystalline semiconductor film, a polysilicon film, or a microcrystalline semiconductor film.
When evaluating a TFT, the most important characteristic is reliability. Within the problem of reliability, the largest is that an alkaline metal (periodic table group element), a mobile ion, mainly sodium (Na), becomes mixed in. The mixing-in is detected as a phenomenon in which Na is electrified to have a positive electric charge and Vth changes by Na moving as an ion throughout the film, preventing the practical use of the TFT. The following can be given as examples of this type of impurity to (hereafter, impurities such as Na which cause the reliability of a TFT to drop are referred to as contaminating impurities throughout this specification): alkaline metals (periodic table group 1 elements) and alkaline earth metals (group 2 elements), such as sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), and barium (Ba). The reduction of these contaminating impurities is indispensable for the manufacture of reliable TFTs. However, contaminating impurities get mixed with TFTs from a variety of impurity sources, such as gases in the atmosphere or high pressure gas cylinders, glass substrates, and manufacturing apparatus such as a sputtering device. In particular, the contamination from a glass substrate is a serious problem, and even by using a glass substrate with a Na composition of 0.1% or lower, this reliability problem has not been solved. Therefore, a blocking film such as a silicon nitride film is formed on the substrate, preventing contaminating impurities contained in the glass substrate from diffusing, and preventing a lowering of reliability.
However, as a result of analyzing the contaminating impurity concentration in a TFT, the contaminating impurity concentration of the interface between films structuring the TFT is between 5×1016 atoms/cm3 and 5×1019 atoms/cm3, higher in comparison to the contaminating impurity concentration within the films (generally 1×1016 atoms/cm3 or less), identifying the cause of the reduction in TFT reliability. In particular, the fact that a contaminating impurity exists in the interface between a semiconductor film and an insulating film in contact with the semiconductor film (an insulating film which functions as a gate insulating film (hereafter referred to as a gate insulating film), an insulating film which functions as a blocking film, or an interlayer insulating film), or in the interface between the gate insulating film and a film which contacts the gate insulating film (such as the semiconductor film, a gate wiring (this includes a gate electrode throughout this specification), or an interlayer insulating film), is a major cause of the harm to TFT reliability.
Note that the impurity concentrations throughout this specification are concentrations measured by performing an analysis in the depth direction by using secondary ion mass spectroscopy (hereafter referred to as SIMS). A SIMS analysis is a method in which a primary ion is irradiated onto a test sample, and a mass analysis is performed on secondary ions emitted from the test sample surface and from a depth of several angstroms. SIMS analysis is characterized by high detection sensitivity and the ability to analyze microscopic regions. However, an analysis using SIMS is performed by increasing the current density of the primary ion while sputtering the surface, and therefore there is a limit to the resolution ability in the depth direction. Therefore it is difficult to perform accurate measurements of the element concentration in the film interface, and SIMS analysis is actually done in succession for a first film and then for a second film in contact with the first film, measuring the element concentration in the interface between the first film and the second film, and in the neighboring area (to several angstroms). In the present specification, the concentration in the interface between the first film and the second film, and in the neighboring area (to several angstroms) is taken as the element concentration in the interface between the first film and the second film.
An example is shown in FIGS. 4 to 6B in which sodium (Na) exists in the interface between a gate wiring and a gate insulating film. FIGS. 4 and 5 show the result of SIMS analysis of a TFT. A SIMS analysis result before BT processing (bias temperature: heating while applying a voltage) is shown in FIG. 4, and a SIMS analysis result after BT processing is shown in FIG. 5. Note that the minimum detection level, or the background level, of Na in FIGS. 4 and 5 is approximately 1×1015 atoms/cm3.
Only one peak is observed showing the existence of Na in FIG. 4 (before BT processing). This is peak A seen in a location corresponding to the interface between the gate wiring and the gate insulating film, and the neighboring area. However, two peaks are observed showing the existence of Na after BT processing, as shown in FIG. 5. One of these peaks is the peak A, also shown in FIG. 4 (before BT processing), seen in the location corresponding to the interface between the gate wiring and the gate insulating film, and the neighboring area. The other peak is a peak B seen in a location corresponding to the interface between the gate insulating film and a semiconductor film, and its neighboring area, and is not seen in FIG. 4 (before BT processing). It is thus understood from FIGS. 4 and 5 that Na moves within the gate insulating film due to BT processing. As a result, changes are seen in the ID-VG characteristics from before BT processing (solid line) to after BT processing (broken line) for both an n-channel TFT (shown in FIG. 6A) and a p-channel TFT (shown in FIG. 6B). This shows fluctuations in threshold voltage (Vth), one of parameters for evaluating TFT characteristic, and it shows a result in which the TFT reliability is harmed.